1. Field of the Invention
The present invention relates to an antenna for a portable terminal used in a satellite communication or a ground mobile radio communication and particularly, to a helical antenna.
2. Description of the Prior Art
A helical antenna based on a conventional technique will be first explained with reference to FIG. 6 which is a perspective view showing a conventional helical antenna disclosed in Japanese Laid-open Patent Application No. Hei-7-202551.
The conventional helical antenna is designed in such a structure that helical conductors 103, 104 and helical conductors 105, 106 are helically wound around two coaxial cables 101, 102 having different lengths through supports 107, respectively. In this structure, the length of coaxial cable 101 is set to be larger than that of coaxial cable 102, and power is supplied to helical conductors 103, 104 through U balun 108 at the upper end of coaxial cable 101. The dimension of coaxial cable 102 is set so that the tip of coaxial cable 102 extends to the lower side of the winding end of helical conductors 103, 104, and power is supplied to helical conductors 105, 106 through U balun 108.
In this case, the group of coaxial cable 101 and helical conductors 103, 104, and the group of coaxial cable 102 and helical conductors 105, 106 operate as independent helical antennas. In FIG. 6, reference numeral 110, 111 represents a connector, and reference numeral 109 represents a radome.
Accordingly, in the case that each of these antennas is used as an antenna for a satellite communication terminal and a transmission frequency band and a reception frequency band are separated from each other, these antennas may be adjusted so that one of these antennas is used as an antenna for transmission and the other antenna is used as an antenna for reception. As explained above, the conventional antenna is usable in a wide frequency band because the antenna is designed in the two-stage structure.
In the conventional technique as explained above, the two independent helical antennas are piled up in the two-stage structure, and thus it has an effect of widening the frequency band, however, there is a disadvantage that the entire size of the helical antenna is large.